Manufacturing thin monocrystalline layers



United States Patent 3,385,737 MANUFACTURING THIN MONOCRYSTALLINE LAYERS Bertrand Alain Dreyfus, Sevres, Seine-et-Oise, France, as-

signor to Societe dElectronique et dAutomatisme, Seine, France No Drawing. Filed July 8, 1964, Ser. No. 381,264 Claims priority, application France, July 15, 1963, 941,415, Patent 1,370,724 2 Claims. (Cl. 148-1.6)

ABSTRACT OF THE DISCLOSURE Thin monocrystalline layers of semi-conducting materials, e.g., germanium, are produced by controlled condensation of monoatomic vapors of the semi-conducting material upon an amorphous substrate to create a layer of "between about 300 A. to some tens of microns thickness.

Background of the invention An object of the invention is to provide a method for obtaining such layers on amorphous substrate, in contradistinction with the use of monocrystalline substrata which, in certain conditions, mainly concerning the choice of materials and operative temperatures, enabled the growing of a thin layer monocrystal from an epitaxy phenomenon between the substratum crystal and the growing semi-conductor crystal.

A further object of the invention is to provide such a method without any recourse to a fractioned crystallization and consequently based on phenomena which were diflicult to control.

According to a feature of the invention, the method of manufacturing thin monocrystalline layers on amorphous substrata comprises the step of condensing on such a substratum a mono-atomic vapour of the constitutive element or elements of the layer under such conditions, specially under such temperature conditions, that the layer grows as a monocrystalline one from the deposition of said atoms of vapour on said substratum.

According to a further feature of the invention, said mono-atomic vapour is generated within a conditioned volume from a thermal process of evaporation under vacuum of the element or elements of the layer to obtain, or from a thermal process of dismutation, dissociation or reduction of compounds of such element or elements, the temperature of such process being so provided as to avoid the formation of molecules in the vapour phase.

According to a further feature of the invention, and in case there is an incertitude in the production of only atoms in the said vapour, the method may include an intermediate step between the formation of the vapour and its condensation on the said substratum, which intermediate step comprises either the separation of parasitically formed molecules from the mono-atomic vapour or the cracking of such molecules Within said vapour.

According to another feature of the invention, the temperature conditioning of the substratum on which develops the monocrystalline thin layer from deposition of atoms of said vapour is such that only a very small number of crystal germs are formed during the early time of the condensation, whereby the layer will grow from such a very small number of germs and will be of the same order of coherence as that obtained in massive crystals of the same elements which have been grown according to conventional methods for such massive crystals.

According to yet another feature of the invention, and as an alternative to such a growth starting from a small 3,385,737 Patented May 28, 1968 number of germs, the temperature conditioning of the condensation of the vapour on the substratum is provided such that it is substantially equal to the temperature of equilibrium between the vapour and solid phases or between a liquid phase which would be intermediate between the vapour and solid phase and said solid phase, according to the pressure under which the vapour is treated for deposition on the substratum, though being slightly under such an equilibrium temperature, whereby a first layer of atoms is formed on the substratum which is particularly compact and from which issues a frontal growth of the thin monocryst-alline layer.

In order to further disclose the invention, reference will be made to the manufacturing of thin monocrystalline layers given as illustrative examples only, and for instance of thin monocrystalline layers of germanium and silicon; such examples will be directly applicable to other types of semi-conductors such as for instance compounds of the III-V kind (III and V refer to the classification of elements), or else and for instance also to tungsten from which examples may be derived the method of formation of other metallic elements or compounds.

First example.production of a thin monocrystalline layer of germanium. The germanium is evaporated at a temperature of at least 1200 K. Said temperature is the one of the vapour generator, a crucible for instance, placed within a vessel within which has been made a vacuum of the order of 10- Torricelli. At such temperature, it is highly probable that all vapours are monoatomic and consequently it may be considered also valid for any other element of the enumeration given above. In the present example as in the other ones, several crucibles may be provided within the vessel, with different elements therein, for doping the basic semi-conductor material with wanted impurities, the openings of said crucibles being provided for obtaining a required mixture of the elements in their vapour phases.

For avoiding that molecules are formed in said vapour, and to dissociate by cracking process the molecules which might have been formed by the evaporation, the vapour is passed through a cylindrical oven the temperature of which is maintained at a high value, for instance of the order of 1200 F., so that the flow of mono-atomic vapour is carried up to the substratum on which the atoms will condense.

This substratum is afiixed to a carrier the temperature of which may be lowered, for instance from a circulation of refrigerating liquid therethr-ough, such as water or liquid air or any other agent in this respect. Consequently the substratum is brought and maintained at a temperature for which the speed of germination is low whilst preserving the possibility of growth of the crystal. Such conditions of germination and growth are well-known per themselves for massive crystals, For the germanium, said temperature is comprised between 250 and 350 C. substantially. The homogeneity of the temperature of the substratum must be rather good and present a gradient of temperature as low as possible between the middle point and the edge, for instance lower than 10 C. per centimeter in order to obtain a layer of homogeneous crystallization.

In such conditions, a very small number of germs are initially formed on the substratum whereon condenses the mono-atomic vapour and it is from such germs that the crystal grows in a thin layer. Such layer is quasimonocrystalline and only comprises when formed, a small number of defectuosities than are present, in same nastance. For such a speed of growth, the substratum is maintained at an appreciable distance from the vapour generator, for instance spaced from 12 to 25 centimeters, or more, such spacing e-asing the intercalation of the above mentioned over when advantageous between said generator and said substratum.

Second example-Production of a thin monocrystalline layer of germanium, starting from evaporation under vacuum in the preceding conditions. The second step, condensation, is controlled with a temperature of the substratum maintained some degrees lower than the equilibrium temperature point between the vapour and solid phases, or between the liquid and solid phases, according to the pressure created within the vessel. On the amorphous substratum then is created a first layer of atoms of a very compact organization since the atoms arrive on the substratum one per one and have a kinetic energy sufiicient for organizing themselves in a state of minimum energy, hence such a compact assembly. Thereafter, the layer grows frontally, at a slow speed since the temperature is quite near to the equilibrium solid/vapour states or liquid/vapour states. Such a frontal growth may be compared to the one obtained from the liquid phase by the Czochralsky method for instance for massive crystals, such method being well known per se, in order to more clearly visualise how the growth of the monocrystalline layer goes on in the formation of the said thin layers.

Third example-The mono-atomic vapour of germanium is produced from germanium dioxide the vapour of which is heated to the dismutation temperature, comprised between 325 to 400 C. approximately. The substratum is heated at about 325 C. It must be noted that the temperature given in the first example is compatible with the dismutation process. This however, is not true for all cases-for instance, when instead of germanium dioxide, one uses germanium dichloride, dismutation occurs only about 675 C., a temperature not compatible with any temperature provided for the substratum in the two preceding examples.

When the temperatures are compatible, as for the ger manium dioxide, a quasi-monocrystalline layer is obtained whereas the germanium tetrachloride in vapour phase is evacuated by any suitable means.

Fourth example.Obtention of a monocrystalline thin layer of silicon from silane. Silane is dissociated in a temperature range comprised between 900 C. and 1420 C. which is the melting point for silicon. The process is controlled in hydrogen atmosphere, consequently in a diluted environment, with a pressure of about 0.5 to 20 Torricelli for the silane. The thermal decomposition of the silane results in a men-atomic vapour of silicon. The substratum is brought to a temperature between 1200 C. and 1420" C. which is the temperature of equilibrium between the solid phase and the uncondensed phase. Said temperature is well known for the three dimension solids, but it may depend on operative conditions and must be adjusted in relation to first results empirically obtained in each case.

In this fourth example, it must be noted that the temperature of the substratum is controlled for carrying into practice the second mode of crystallisation which has been described above and that such a temperature is compatible with the temperature of decomposition of the silane.

Fifth example.A refractory metal such as tungsten can be obtained as a thin monocrystalline layer from an infrachloride of said metal by a dismutation process. Starting from hexachloride of tungsten, infra-chlorides are obtained by passage of the gas over metal tungsten heated to a temperature comprised between about 300 C. to 400 C. Such infra-chlorides dismutate over about 1300 C. From empirical results of applicant, the temperature of crystallization of a thin solid layer of tungsten situates near 1300 C. Said temperature is applied to the substratum and it is compatible with the temperature of dismutation.

A similar result is obtained for the molybdenum and more generally for any refractory metal.

For ensuring the separation of parasitic molecules from the atoms, when required, instead of using an oven as disclosed in the first example above, one may use a separation by mechanical priocess, i.e., a kinetic separation or by electromagnetic system provided all particles have been previously ionized in the vapour.

In the preceding disclosure, one must understand as being a thin monocrystalline layer any monocrystalline layer of a thickness comprised between about 300 A. and some tens of microns, comprising from some tens to some thousands of defectuosities of the dislocation type per square centimeter.

Iclaim:

1. A method of manufacturing thin monocrystalline layers of semi-conductors and refractory metals which comprises evaporating in a closed zone metal constitutive of the layer located in said zone and heated to a temperature of at least 1200 K. while said zone is maintained under a vacuum of about 10 Torricelli creating an atmosphere of mono-atomic vapors of said metal, condensing said metal in said zone onto an amorphous substratum maintained in said zone at a temperature slightly lower than the temperature of equilibrium of monoatomic vapor and solid phase of said metal, the substratum being maintained with a homogeneity of temperature such that the substratum does not present a temperature gradient above 10 C. per centimeter between its middle point and its edge and continuing said contact of said temperature controlled substratum with said atmosphere until a thin layer of monocrystalline metal having a thickness of at least 300 A. is formed.

2. A method of manufacturing a thin monocrystalline layer of germanium which comprises evaporating in a closed zone germanium metal located in said zone and heated to a temperature of at least 1200 K. while said zone is maintained under a vacuum of about 10- Torricelli creating an atmosphere of monoatomic germanium vapors and condensing germanium in said zone onto an amorphous substratum maintained in said zone at a temperature between 250 to 350 C. with a homogeneity of temperature such that the substratum does not present a temperature gradient above 10 C. per centimeter between its middle point and its edge until a thin layer of monocrystalline germanium having a thickness of at least 300 A. is formed.

References Cited UNITED STATES PATENTS 2,759,861 8/1956 Collins et a1. 148-174 2,850,414 9/1958 Enomoto 148-1.6 2,880,117 3/1959 Hamlet 148-175 2,994,621 8/1961 Hugle et a1. 148-174 3,012,902 12/1961 Bayer 148-174 3,063,871 11/1962 Barkemeyer et a1. 148-174 3,128,854 4/1964 Bean et a1 148-1.6 3,139,361 6/1964 Rasmanis- 148-175 3,160,521 12/1964 Ziegler et a1 148-175 3,160,522 12/1964 Heywang et al 148-174 DAVID L. RECK, Primary Examiner.

HYLAND BIZOT, Examiner.

N. F. MARKVA, P. WEINSTEIN, Assistant Examiner's. 

